President Bill Clinton campaigns for his wife during a campaign event at Milford Junior High School in New Hampshire Photo: Getty

At a rally in a school Mr Clinton said his wife's supporters had suffered "unbelievable personal attacks" on the internet from those of Mr Sanders.

He said: "People who have gone online to defend Hillary, to explain why they supported her, have been subject to vicious trolling and attacks that are literally too profane often, not to mention sexist, to repeat."

Mr Clinton was referring to the so-called "Berniebro" phenomenon in which a small number of Mr Sanders' supporters have harassed those who disagree with them.

According to polls of Democratic voters in New Hampshire 64 per cent of women under the age of 45 support Mr Sanders, and only 35 per cent support Mrs Clinton, who would be America's first female president.

Former U.S. Secretary of State Madeleine Albright (L) introduces Hillary Clinton during a campaign stop in Concord, New Hampshire Photo: Reuters

In an attempt to sway women Mrs Clinton had Madeleine Albright, the first female US Secretary of State, appear at a rally.

Mrs Albright said: "A lot of you young women think it's done. It's not done. Hillary Clinton will always be there for you. Just remember there's a special place in hell for women who don't help each other."

Gloria Steinem, the feminist icon, suggested young women were only going to Mr Sanders' rallies so they could find boys to date.

She said: "When you're young, you're thinking 'Where are the boys? The boys are with Bernie'."

Both interventions backfired on Mrs Clinton as many young women said they were "insulted" to be told they should vote purely on the basis of their gender.

Meanwhile, Mr Clinton extended his assault on Mr Sanders, calling him hypocritical fro making attacks on Wall Street and "the billionaire class".

The former president referred to reports that Mr Sanders had taken part in Democratic Party fundraising events where wealthy donors gave more than $30,000 a year each.